Force Bursts (フォルス・バースト?) or FB for short, are the party character's signature moves. To activate the move, the gauge in the upper left corner should be filled at how many bars required per move in order to use them.
The gauge has three bars, and it builds up whenever you use skills. It doesn't empty between battles.
FB takes up a certain amount of bars from the gauge, up to all three bars at once. As your entire party shares the gauge, think wisely before using a move that consumes the whole gauge.
Only characters that have Great affinity with a Ragnafact can use its FB skills.